Nowadays, many ICs comprise sensor functionality, as the demand for greater multifunctionality of the IC increases. Parameters to be measured by such sensors may include temperature, relative humidity, moisture, presence and/or concentration of a chemical compound in a sample, and so on. Such sensors find applicability in a wide range of application domains, including medical devices and assays, automotive, smart homes, food packages and more. It is of course commercially interesting to integrate sensor functionality into an IC as it reduces the complexity and foot print of the overall sensing system, and can mean a significant cost saving over discrete components.
To this end, it is desirable that additional sensing functionality can be added to an IC without having to alter the IC manufacturing process. This may often prove practically impossible, in which case this objective translates into adding the smallest number of alterations as possible to the standard process flow to limit the added cost of the required functionality to the IC design.
A particular example of sensing functionality that can be added to an IC design is a thermal conductivity sensor. Such a sensor may be used to determine the composition and/or pressure of a fluid such as a gas or a liquid. The principle of such a sensor is for instance explained in a paper by Nicholas R. Swart et al. in Electron Devices Meeting, 1994, IEDM '94, Technical Digest, International on pages 135-138. In this paper, a polysilicon coil-based micro-Pirani gauge is disclosed in which a pair of meandering polysilicon coils is formed in a silicon substrate of a CMOS IC. The heat transfer from the active to the passive coil is measured, with the amount of heat transfer being governed by the thermal conductivity of the gas in between the two coils.
The sensing coil essentially is operated by the determination of the resistance of the coil at a given current through the coil, which is temperature-dependent. The determination of the resistance and hence the temperature of the coil can be used to determine the heat transfer from the active coil to the passive coil, as R=R0(1+a(T+T0)), in which R0 is the absolute resistance of the coil at temperature T0, R is the measured resistance, a is the temperature coefficient of the resistance and T is the actual temperature at which R has been determined.
The micro-Pirani gauge disclosed by Swart et al. suffers from a number of drawbacks. Most notably, the formation of the coils in the substrate compromises the integrated density of semiconductor devices of the IC, which is not ideal when silicon real estate comes at a premium. Moreover, the fact that both coils are formed of polysilicon, i.e. have the same temperature coefficient of resistance, limits the sensitivity of the thermal conductivity sensor.